Although a large proportion of patients with obesity, type 2 diabetes and/or dyslipidemia develop steatosis (non-alcoholic fatty liver disease or NAFLD), only a small percentage of these individuals experience progressive liver disease, such as non-alcoholic steatohepatitis (NASH) and cirrhosis. Therefore, an issue of primary importance to the understanding of NAFLD remains the study of the cellular and molecular events that change the natural history of the disease from non-progressive steatosis to NASH and progressive liver disease. Our work has focused on the hypothesis that saturated fatty acids are an important determinant of disease progression in NAFLD via disruption of endoplasmic reticulum homeostasis. Data in this application demonstrate that there is a significant, positive relationship between saturated fatty acids in the liver and both liver injury and ER stress markers in morbidly obese human subjects. Using cell culture systems and dietary models of NAFLD we demonstrate that saturated fatty acids promote ER stress, liver cell death and liver injury. We have identified an ER membrane- bound protein, SERCA truncated isoform (S1T), that is upregulated by saturated fatty acids and mediates saturated fatty acid-induced ER stress and cell death, in part, via redistribution of luminal calcium stores. Thus, we hypothesize that the composition of fatty acids delivered to or stored within the liver are an important determinant of ER homeostasis and susceptibility to liver injury in NAFLD. The specific aim of this application is to elucidate how saturated fatty acids provoke ER stress and whether and how ER stress may be linked to loss of liver cell integrity. The primary hypotheses to be tested are that 1) S1T and CCAAT-homologous protein (Chop) mediate saturated fatty acid-induced ER stress and impaired liver cell integrity;and 2) saturated fatty acids impair the folding and degradative capacities of the ER and thus the ability of the UPR to reestablish ER homeostasis following ER stress. Identification of the cellular mechanisms that determine whether excess lipid delivery to and accumulation in the hepatocyte is tolerated or cytotoxic will provide information fundamental to our understanding of disease progression in NAFLD.